Over the past half century, viruses have been classified and identified by such characters as the shape, size and serological specificity of their virions, and also by biological characters such as host and vector type. These features placed viruses into seemingly sensible genera and other taxa, but the evolutionary relationships of the genera (and higher taxa), and the processes that produced them, were unknown. However, knowledge of viral genomic sequences is starting to reveal the wealth of genetic processes involved in viral evolution, and the relative importance of each (Fig. 6.1). Sequence information has confirmed most of the earlier groupings, but also revealed a much greater shared genealogy than expected.
Taxonomic comparisons show that the genes of closely related viruses, such as isolates of a single species, usually only differ from one another by point mutations and, occasionally, by insertions or deletions of one or a few nucleotides. The different species of each viral genus differ by a greater number of changes of the same type. Thus mutation is a primary evolutionary driving force for viruses just as it is for all other organisms.
Comparisons of the genes of viruses placed in separate genera show that some are related, some are not. For example, all viral RNA replicase genes encode proteins that share sequence motifs, and hence may be related (Poch et al., 1989). Likewise, the virion proteins of most viruses with isometric virions of 25-30 nm diameter have a characteristic eight-stranded (3-barrel structure (Rossmann & Johnson, 1989) the only known exception being those of Q(3 levivirus, which has virion proteins with a (3-sheet/a-helical hook structure (Valegaord & Liljas, 1990).